The present invention concerns detecting intruders by emitting and receiving ultrasound waves in an automobile vehicle.
With conventional ultrasound detection techniques the vehicle contains an ultrasound field which is a combination of the waves emitted and reflected inside it. This field is stationary or quasi stationary when the environment of the vehicle is calm or at least not subject to major modifications.
Movement is generally detected by amplitude or phase demodulation of the ultrasound wave received at the receiver and then analyzing the demodulated wave in accordance with a number of cumulative criteria, such as the amplitude, the duration and the frequency of the detected signal.
For example, intrusion may be considered to be characterized by the presence of a signal level above a given threshold for frequencies below 8 Hz (low frequencies) or for frequencies in the range 10 Hz to 20 Hz (high frequencies).
However, the above intrusion detection techniques cannot totally avoid the risk of false alarms.
In particular, thermal shock to which the vehicle may be subject generates low frequencies in the demodulated signal and can be confused with intrusion. Similarly, impacts to the roof or the windows of the vehicle result in high frequency modulation in the analyzed signal and therefore can also be erroneously confused with intrusion.
Another important problem encountered with techniques employing amplitude demodulation relates to the fact that they necessitate a variable gain given that the signal is dependent on the level of the carrier and that if the carrier is saturated information is lost.
Document DE 38 11 113 A discloses a detection method and a detector device in which ultrasound waves received are detected by quadrature demodulation and the rotation of the vectors whose components are the pairs of demodulated signals obtained is analyzed.
However, a device of the above kind is essentially intended for detecting intruders in dwellings and would not be suitable for the specific environment of an automobile vehicle.
In this specific environment various kinds of phenomena occur that must not be considered to be intrusion, including:
isolated or repeated light impacts on various places on the bodywork or the windows of the vehicle, PA1 vibration of low amplitude but of long duration, cause for example by the starting up of the cooling system or the air conditioning system when the vehicle is stopped, PA1 the presence of insects in the passenger compartment, and PA1 thermal shock to which the vehicle is subjected. PA1 the received signal is quadrature demodulated by mixing it with two signals with a relative phase difference of .pi./2 and having the same frequency as the carrier of the emitted ultrasound waves to obtain a pair of demodulated signals, PA1 the absolute value of each of the demodulated signals of each pair of signals obtained is compared to a respective threshold value to determine pairs of valid signals if the absolute value of at least one of the two demodulated signals is higher than the respective threshold value, and invalid pairs otherwise, PA1 a process of counting the number of successive rotations of a vector whose coordinates correspond to the values of the successive valid pairs of signals is processed, PA1 for each invalid pair, the value of the count of the number of rotations is progressively reduced, and PA1 the tripping sounding of an alarm is commanded if the value of the count of the number of rotations reaches a given threshold. PA1 the value of the count of the number of rotations is reduced by multiplying that value by a coefficient in the range 0 to 1. PA1 the value of the multiplier coefficient is chosen according to the required degree of immunity to noise. PA1 a bidirectional counting process is employed, depending on the direction of rotation of the vector, and the tripping of an alarm is commanded if the absolute value of the count of the number of rotations reaches a given threshold. PA1 the demodulated signals are sampled and converted into numerical values, a plurality of identical quadrants are defined in the plane of rotation of the vector and a counter is incremented up or down if the vector whose coordinates correspond to the numerical values of the demodulated signals passes from one quadrant to another by rotating in one direction or another. PA1 there are eight quadrants. PA1 the demodulated signals are filtered to eliminate variations in the signal having a frequency greater than half the sampling frequency. PA1 the comparator means, the counting means and the command means comprise one and the same microcontroller at the input of which the demodulated signals are sampled and converted into numerical values. PA1 the means for reducing the value of the count comprise multiplier means adapted to multiply the value of the count by a coefficient in the range 0 to 1. PA1 the device further comprises time-delay means for selectively switching into service during a predetermined first period at least the processing and counting means, for maintaining said means in service beyond said predetermined first period for as long as a variation in the count of rotations is established, and for switching said means out of service during a predetermined second period at the end of said predetermined first period if no variation in the count has been established during said first period. PA1 said processing and counting means are maintained in service if the number of rotations determined at the end of said predetermined first period is higher than a threshold.
The above phenomena are characterized by received waves having highly diverse characteristics and a principal aim of the invention is to detect intrusion by emitting and receiving ultrasound waves in an automobile vehicle with the capability for good discrimination between phenomena of the above kind and actual intrusion into the vehicle in order to minimize the risk of false alarms.